1. Field of the Invention
The present invention relates to a thermomagnetic recording method. More specifically, the present invention relates to a thermomagnetic recording method which is easy to control and which allows stable recording of information by applying an external magnetic field to balance the forces acting on recorded bits, so that the recorded information may be kept stable even if the magnetic wall coercive force is very low or substantially zero.
2. Brief Description of the Prior Art
A bit recording method which records by focusing a laser beam is known as the conventional thermomagnetic recording method. The bit recording method includes the Curie temperature recording method and the temperature compensation point recording method. With these methods, bits can be formed when the laser beams are incident thereon. However, considering the various factors acting on the bits, the bits are unstable and recorded bits may disappear if the magnetic wall coercive force is low. Therefore, a high magnetic wall coercive force is required to retain the recorded bits as recorded information.
It has also been proposed to form fine bits in an amorphous GdCo thin film. However, bits recorded by this method are also reported to be unstable. It has, therefore, been proposed to vary the magnetic characteristics of the amorphous GdCo thin film in the direction of its thickness in order to stabilize the bits. Such techniques are reported in Appl. Phys. Lett. 32(10), pp. 673 to 675. This literature discloses a method for forming a thin film by continuously reducing the sputtering current and a method for forming a bilayered thin film. This literature reports that stable bit information was obtained with films obtained by this method when the temperature and external magnetic field were varied. This literature also reports that bits were stable even if an external magnetic field of 0 Oe or a relatively intense magnetic field such as 280 to 350 Oe was applied to an amorphous GdCo thin film prepared by this method. However, with an amorphous GdCo thin film, stable bit information cannot be held unless the film has a relatively high coercive force. Since the magnetic wall coercive force is relatively high and the film quality is not uniform, it is considered that formation of bits of uniform diameter is difficult. In order to form bits of uniform diameter, the output of the laser must be controlled, resulting in a complex structure of the laser.
The literature mentioned above also reports a case of an amorphous GdCo thin film formed under constant sputtering conditions. With this thin film, it is either impossible to form bits of small diameter or bits are extremely unstable against changes in the external magnetic field and temperature. Bits were formed by the temperature compensation point recording method in this case. The compensation temperature is only slightly higher than ambient temperature and is subject to abrupt changes with changes in the vicinity of ambient temperature. For this reason, temperature control within a limited range is necessary for the purpose of holding the recorded information.
It has also been proposed to record a large quantity of information on a ferromagnetic thin film of MnBi, Ti-substituted MnBi, and EuO by the Curie temperature recording method (IEEE Trans, Magn., Vol. MAG-13, May of 1977, pp. 982 to 988; Applied Optics, Vol. 13-4, pp. 770 to 777). According to this literature, MnBi is most preferable since the recording density is high and the angle of magnetic polarization is great. However, MnBi has a Curie temperature as high as 360.degree. C. and requires more energy for recording. On the other hand, the decomposition temperature is as low as 450.degree. C., providing only a slight difference from the Curie temperature. For this reason, it is difficult to control the output of the laser so that it may fall within the range of these two temperatures. Furthermore, an MnBi thin film has low weathering or weather resistance and is easily damaged by a humid atmosphere. MnBi has a coercive force of 1.5 kOe and requires a magnetic field of 700 Oe for erasing the information. Thus, a separate device is required to induce such a high magnetic field for erasing the recorded information. This adversely affects the size of the device.
It has also been proposed to use an Mn-Cu-Bi alloy having a low Curie temperature of 200.degree. C. as a thermomagnetic recording medium (Ibid., IEEE Trans. Magn.) However, an Mn-Cu-Bi alloy has a coercive force as high as 1.5 kOe. Furthermore, with this alloy, the diameter of the bit varies with the output of the laser. In other words, the greater the pulse width of the laser beam, the greater the diameter of the bit. This results in degradation in recording sensitivity and impairs high-density recording.
It has also been proposed to record information at high density and with low power by the compensation temperature recording method using a thin film of (BiGaLuSm).sub.3 (FeAl).sub.5 O.sub.12 on GGG (gallium garnet) substrate by the liquid phase epitaxial method (to be referred to as the LPE method hereinafter). This technique was proposed in Transactions of the Japan Society of Applied Magnetics, Nov. of 1980, 5aB-3. In this case, the output of the laser is similarly as high as 70 to 100 mW. Furthermore, cylindrical magnetic bubble domains have star shapes instead of circular shapes. This results in degraded readout sensitivity. The diameter of the bit is also too great which is not preferable. The coercive force of the thin film is not sufficiently low from a practical point of view.